This invention relates generally to an apparatus and method for multiplexing an array of resistor heaters in a thermal inkjet printhead, and more particularly to an apparatus and method for multiplexing an array of resistor heaters to form a sparse array for limiting current flowing to non-addressed resistors.
Thermal ink jet printers and are well-known in the an, as described in U.S. Pat. Nos. 4,490,728, and 4,313,684. A thermal ink jet printer includes an array of printing cells formed on a printing head, which in turn is mounted on a replaceable, disposable housing having one or more ink reservoirs. In the following discussion, an array of resistor heaters will be portrayed schematically as a rectilinear matrix made up of rows and columns of resistor heaters. Each printing cell includes a small ink reservoir, a printing nozzle, and an electrically driven resistor heater formed opposite the printing nozzle. The resistor heater of each cell is connected to a current source by an address lead and a ground lead. The printing cells and required electrical leads are typically formed on a silicon substrate by known photolithographic deposition, metallization and etching techniques. In operation, the printing cell is fired by switching one or both of the address and ground leads to direct a current through the resistor heater. The heat generated by the current in the addressed resistor heater vaporizes a portion of the ink in the reservoir, ejecting a drop of ink through the printing nozzle onto a medium such as a sheet of paper.
The performance and print quality of a thermal ink jet printhead can be enhanced by increased the printing cell density on the printhead. Printing cell density on the printhead is limited in part by the space occupied by conductive leads which electrically connect the array of printing cells to the control circuitry of the printer. Printing cell density could therefore be increased if fewer conductive leads were required to electrically connect the array.
FIG. 1 depicts one known arrangement for reducing the number of conductive leads in which each resistor heater is connected to a separate address lead 10, while each ground lead 12 is connected to multiple resistor heaters R. This arrangement requires 54 leads for 50 resistor heaters, a relatively high number.
Turning now to FIG. 2, a fully multiplexed array, shown generally at 20, requires the fewest number of leads. A fully multiplexed array is one in which all the resistor heaters 22 in each row are connected to a single address lead 24, and all the resistor heaters in a given column are connected to a single ground lead 26. In this way, the total number of leads required for an array of any given size can be reduced to a minimum. For example, only 10 address leads and 10 ground leads are required for a fully multiplexed 10.times.10 array of 100 resistor heaters. As the aspect ratio of an array varies from unity, the degree of reduction is less, but remains significant. For example, a multiplexed 5.times.20 array of 100 resistor heaters requires as few as 25 leads, which is still a significant reduction over the 100 or more required in a non-multiplexed array.
Multiplexed arrays suffer from one significant drawback however. Due to the interconnection of address leads and ground leads through multiple resistor heaters, the firing of an addressed resistor heater results in parasitic voltages being impressed upon non-addressed resistor heaters, driving leakage currents through them. In FIG. 2, for example, when resistor heater 22a is fired, parasitic voltages are impressed upon surrounding resistor heaters, driving leakage currents through them.
Leakage currents cause several problems. First, leakage current levels through a particular resistor heater can reach the turn on energy (TOE) of the resistor heater, causing it to misfire. Even if a leakage current does not reach TOE, it will tend to raise the temperature of the unaddressed resistor heater and render precise control of the drop size ejected from the printing cell difficult. Leakage currents also increase the total current flowing through each ground lead, requiting larger and more expensive ground lead switching transistors. Finally leakage currents cumulatively increase the power delivered to the printhead. The increased power levels can raise the printhead temperature and change the size of ink drops ejected from individual printing cells, adversely affecting print quality.
One method for nullifying parasitic voltages in multiplexed arrays includes biasing the ground leads in the array. While effective to control leakage currents, this method requires additional switching to open circuit the ground lead bias voltage when the resistor heater is addressed. The additional switching step requires switches and control elements which add cost and complexity to the printer. Another known method used to nullify parasitic voltages incorporates a transistor interposed between the address line and the supply side of each resistance heater. The turn-on voltage of the transistor is greater than the maximum parasitic voltage, thereby isolating non-addressed resistors from ground. While solving the problem of parasitic voltages and leakage currents, this method also adds significant complexity and expense to the fabrication of the printhead.
Accordingly, a need remains for a cost effective method of multiplexing an array of resistor heaters on a thermal inkjet printhead which effectively controls parasitic voltages and leakage currents in the array.